At present, a contact charging method has been put into practical use as one of methods for charging the surface of an electrophotographic photosensitive member.
The contact charging method is a method in which a voltage is applied to a charging member disposed in contact with the electrophotographic photosensitive member, to cause micro-discharge at the contact part between the charging member and the electrophotographic photosensitive member and the vicinity thereof to charge the surface of the electrophotographic photosensitive member.
In the contact charging method, a method having come into wide use is one in which a voltage created by superimposing an alternating-current voltage on a direct-current voltage is applied to the charging member (hereinafter referred also to as “AC+DC contact charging method”). In the case of the AC+DC contact charging method, a voltage having a peak-to-peak voltage that is twice or more the voltage at which the charging is started is used as the alternating-current voltage.
The AC+DC contact charging method is a method in which stable charging high in charging uniformity can be effected because of the use of the alternating-current voltage. However, insofar as an alternating-current voltage source is used, this method results in a charging assembly and an electrophotographic apparatus which are large in size and an increase in cost, as compared with a method in which only a direct-current voltage is applied to the charging member (hereinafter referred also to as “DC contact charging method”).
That is, the DC contact charging method is superior to the AC+DC contact charging method in respect of minituarizing the charging assembly and electrophotographic apparatus and achieving cost reduction.
The shape of the charging member is commonly roller-shaped (hereinafter the roller-shaped charging member is referred also to as “charging roller”).
In the case of the contact charging method, it is also necessary to sufficiently and uniformly secure a contact nip between the electrophotographic photosensitive member and the charging member, and hence the charging member used in the contact charging method is required to have a low hardness to a certain extent.
For such a requirement, a charging member has been proposed which has a support and an elastic layer (conductive elastic layer) provided on the support.
The elastic layer (conductive elastic layer), however, often contains low-molecular weight components in a relatively large quantity, and hence such low-molecular weight components may bleed out to contaminate the surface of the electrophotographic photosensitive member. The low-molecular weight components may include reaction initiator residues, reaction by-products, raw-material unreacted matter, vulcanizing agents, softening agents, plasticizers and conducting agents.
Accordingly, at present, in order to keep the low-molecular weight components from bleeding out, it is also prevalent that a surface layer is provided on the conductive elastic layer.
For example, Japanese Patent Application Laid-open No. 2001-173641 discloses a technique in which an inorganic-oxide film formed by a sol-gel process is used as the surface layer.